By use of a tuneable Argon Laser, the action spectrum of bilirubin is being measured in-vitro in exposed bilirubin solutions. Both aerobic and anaerobic chloroform solutions as well as aqueous solutions of bilirubin attached to albumin are being studied. The in-vivo action spectrum of bilirubin is being determined by expanding the laser beam and exposing Gunn-strain rats to it. The photocatabolism products are being traced by incorporation of 3H-bilirubin in the laser irradiated solutions and animals. The effectiveness of agar feeding as an adjunct to phototherapy for neonatal hyperbilirubinemia is being determined by comparing the incidence and severity of diarrhea and cholestasis in control and agar supplemented infants. The duration of phototherapy necessary for control of neonatal hyperbilirubinemia is also being determined. The isolation and characterization of novel bile pigments foundin meconium of infants and stools of patients with hepatic flucuronyl transferase deficiency are being investigated. The metabolic pathway by which they are formed is to be indentified.